/*
 * Copyright 2010-2012 Susanta Tewari. <freecode4susant@users.sourceforge.net>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

package bd.org.apache.commons.math.stat.descriptive.summary;

import bd.org.apache.commons.math.exception.NullArgumentException;
import bd.org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic;
import bd.org.apache.commons.math.stat.descriptive.WeightedEvaluation;
import bd.org.apache.commons.math.util.FastMath;
import bd.org.apache.commons.math.util.MathUtils;

import java.io.Serializable;

/**
 * Returns the product of the available values.
 * <p>
 * If there are no values in the dataset, then 1 is returned.
 *  If any of the values are
 * <code>NaN</code>, then <code>NaN</code> is returned.</p>
 * <p>
 * <strong>Note that this implementation is not synchronized.</strong> If
 * multiple threads access an instance of this class concurrently, and at least
 * one of the threads invokes the <code>increment()</code> or
 * <code>clear()</code> method, it must be synchronized externally.</p>
 *
 * @version $Id: Product.java 1244107 2012-02-14 16:17:55Z erans $
 */
public class Product extends AbstractStorelessUnivariateStatistic
        implements Serializable, WeightedEvaluation {

    /** Serializable version identifier */
    private static final long serialVersionUID = 2824226005990582538L;

    /** The number of values that have been added */
    private long n;

    /**
     * The current Running Product.
     */
    private double value;

    /**
     * Create a Product instance
     */
    public Product() {

        n     = 0;
        value = 1;
    }

    /**
     * Copy constructor, creates a new {@code Product} identical
     * to the {@code original}
     *
     * @param original the {@code Product} instance to copy
     */
    public Product(Product original) {

        copy(original, this);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void increment(final double d) {

        value *= d;

        n++;
    }


    /**
     * {@inheritDoc}
     */
    @Override
    public double getResult() {

        return value;
    }


    /**
     * {@inheritDoc}
     */
    @Override
    public long getN() {

        return n;
    }


    /**
     * {@inheritDoc}
     */
    @Override
    public void clear() {

        value = 1;
        n     = 0;
    }


    /**
     * Returns the product of the entries in the specified portion of
     * the input array, or <code>Double.NaN</code> if the designated subarray
     * is empty.
     * <p>
     * Throws <code>IllegalArgumentException</code> if the array is null.</p>
     *
     * @param values the input array
     * @param begin index of the first array element to include
     * @param length the number of elements to include
     * @return the product of the values or 1 if length = 0
     */
    @Override
    public double evaluate(final double[] values, final int begin, final int length) {

        double product = Double.NaN;

        if (test(values, begin, length, true)) {

            product = 1.0;

            for (int i = begin; i < begin + length; i++) {

                product *= values[i];
            }
        }

        return product;
    }


    /**
     * <p>Returns the weighted product of the entries in the specified portion of
     * the input array, or <code>Double.NaN</code> if the designated subarray
     * is empty.</p>
     *
     * <p>Throws <code>IllegalArgumentException</code> if any of the following are true:
     * <ul><li>the values array is null</li>
     *     <li>the weights array is null</li>
     *     <li>the weights array does not have the same length as the values array</li>
     *     <li>the weights array contains one or more infinite values</li>
     *     <li>the weights array contains one or more NaN values</li>
     *     <li>the weights array contains negative values</li>
     *     <li>the start and length arguments do not determine a valid array</li>
     * </ul></p>
     *
     * <p>Uses the formula, <pre>
     *    weighted product = &prod;values[i]<sup>weights[i]</sup>
     * </pre>
     * that is, the weights are applied as exponents when computing the weighted product.</p>
     *
     * @param values the input array
     * @param weights the weights array
     * @param begin index of the first array element to include
     * @param length the number of elements to include
     * @return the product of the values or 1 if length = 0
     * @since 2.1
     */
    @Override
    public double evaluate(final double[] values, final double[] weights, final int begin,
                           final int length) {

        double product = Double.NaN;

        if (test(values, weights, begin, length, true)) {

            product = 1.0;

            for (int i = begin; i < begin + length; i++) {

                product *= FastMath.pow(values[i], weights[i]);
            }
        }

        return product;
    }


    /**
     * <p>Returns the weighted product of the entries in the input array.</p>
     *
     * <p>Throws <code>IllegalArgumentException</code> if any of the following are true:
     * <ul><li>the values array is null</li>
     *     <li>the weights array is null</li>
     *     <li>the weights array does not have the same length as the values array</li>
     *     <li>the weights array contains one or more infinite values</li>
     *     <li>the weights array contains one or more NaN values</li>
     *     <li>the weights array contains negative values</li>
     * </ul></p>
     *
     * <p>Uses the formula, <pre>
     *    weighted product = &prod;values[i]<sup>weights[i]</sup>
     * </pre>
     * that is, the weights are applied as exponents when computing the weighted product.</p>
     *
     * @param values the input array
     * @param weights the weights array
     * @return the product of the values or Double.NaN if length = 0
     * @since 2.1
     */
    @Override
    public double evaluate(final double[] values, final double[] weights) {

        return evaluate(values, weights, 0, values.length);
    }


    /**
     * {@inheritDoc}
     */
    @Override
    public Product copy() {

        Product result = new Product();

        copy(this, result);

        return result;
    }


    /**
     * Copies source to dest.
     * <p>Neither source nor dest can be null.</p>
     *
     * @param source Product to copy
     * @param dest Product to copy to
     * @throws NullArgumentException if either source or dest is null
     */
    public static void copy(Product source, Product dest) throws NullArgumentException {

        MathUtils.checkNotNull(source);
        MathUtils.checkNotNull(dest);
        dest.setData(source.getDataRef());

        dest.n     = source.n;
        dest.value = source.value;
    }
}
